A nonaqueous electrolyte battery such as lithium ion secondary battery has been expected to have a high-energy density. Recently, the positive electrode active materials such as lithium manganate having a spinel structure, lithium iron phosphate having an olivine structure and the like have been developed. These positive electrode active materials are inexpensive and provide a high level of safety. Particularly, lithium iron phosphate having an olivine structure has a stable crystal structure and has a lithium ion-insertion/extraction potential as relatively low as about 3.4 V (vs. Li/Li+). Therefore, it can provide improved life performance.
On the other hand, the negative electrode active materials which have a higher Li− insertion/extraction potential than carbonaceous materials have been developed. Among these materials, lithium titanate having a spinel structure has such an advantage that it has excellent cycle performances and high safety because it is free from variation in volume during charge/discharge reactions.
Then battery which uses lithium iron phosphate having an olivine structure as the positive electrode active material and lithium titanate having a spinel structure as the negative electrode active material has been proposed. Such a battery has been expected to have excellent cycle performances. However, the battery having the above structure has the problem that it is difficult to detect the state of discharge based on battery voltage because a variation in battery voltage during discharging is small.